Still not sure? Click the image below to open a printable page (Adobe PDF) and compare your existing gasket or fitting to the images representing each size.

Sanitary Size	OD of Clamp End	ID of Fitting
1/2"	0.992	0.370
3/4	0.992	0.620
1.0"	1.984	0.870
1.5"	1.984	1.320
2.0"	2.516	1.870
2.5"	3.047	2.370
3.0"	3.579	2.870
4.0"	4.682	3.834
6.0"	6.562	5.782
8.0"	8.602	7.760
10.0"	10.57	9.782
12.0"	12.57	11.760

Related: National Pipe Size - DN Conversion Chart